This relates to imaging systems and, more particularly, to high dynamic range imaging systems that may include high dynamic range image sensors.
Modern electronic devices such as cellular telephones, cameras, and computers often use digital image sensors such as high dynamic range image sensors. Imagers (i.e., image sensors) may be formed from a two-dimensional array of image sensing pixels. Each pixel receives incident photons (light) and converts the photons into electrical signals.
High dynamic range imagers are designed to capture scenes with light ranges that exceed the typical dynamic range of an individual linear pixel or an analog-to-digital converter. The dynamic range of a pixel can be defined as the ratio of a minimum luminance or brightness in an image that causes the pixel to saturate to the brightness in an image at which the pixel achieves a signal-to-noise ratio (SNR) equal to one. The dynamic range of a scene can be expressed as the ratio of its highest illumination level to its lowest illumination level.
Examples of commonly used techniques for capturing high dynamic range images include combining multiple exposures of varying exposure times, utilizing partial reset level techniques, and providing pixels with logarithmic or other non-linear responses. With a multiple exposure technique, an image sensor takes a first long exposure and then takes a second short exposure. The two exposures are then combined into a high dynamic range image. However, because the two exposures are taken at different times, the image sensor is incapable of capturing scenes that include fast moving objects. In addition, the conventional digital image sensors require excessive amounts of storage (e.g., buffer memory or additional in-pixel storage nodes), thereby increasing the cost of devices that include such sensors.
In one conventional arrangement, each of the pixels in a high dynamic range imager includes a capacitor connected through a switch to a floating diffusion node in that pixel. The capacitors in each of the pixels are used in increasing the dynamic range of the imager. However, this type of conventional arrangement requires an additional switch and capacitor in each of the pixels of an imager.
It would therefore be desirable to provide improved high dynamic range imaging systems.